The present invention relates to a control system and method for an automatic transmission for an automotive vehicle, and more particularly to a system and method for controlling a shift between a gear ratio and the next lower adjacent gear ratio, particularly between the highest gear ratio and the next lower adjacent gear ratio, such as an overdrive and a third gear ratio in the case of a four speed transmission.
In the automotive automatic transmissions, a shift is made automatically by actuating one of a plurality of shift valves in response to a throttle fluid pressure variable with an intake manifold vacuum of an engine and a governor fluid pressure variable with a vehicle speed. In the case of a four speed automatic transmission having a highest gear ratio in the form of an overdrive, the same control method holds and a similar shift valve is used to make a shift between the overdrive and a third gear ratio in response to the throttle fluid pressure and the governor fluid pressure and in accordance with a predetermined shift schedule as shown in FIG. 8 wherein a solid line shows shift points from the third gear ratio up to the overdrive (abbreviated as OD) and a broken line shift points from the overdrive down to the third gear ratio.
Referring to FIG. 7, it is shown how the intake manifold vacuum which affects the throttle fluid pressure varies with the variation of the vehicle speed for different throttle opening degree values. It will be noted that when the throttle opening degree is larger than 50%, substantially the same intake manifold vacuum develops at any vehicle speed value as that developed when the throttle opening degree is 100%. This trend is attributable to various exhaust gas purification measures. When the throttle opening degree is between 50% and 100%, a shift between the third gear ratio and the overdrive is made at substantially the same vehicle speed value. This is not desirable as will be understood from the observation of FIG. 8. In FIG. 8, if intake manifold vacuum versus vehicle speed variation curve .alpha. for throttle opening degree of 50% is drawn, it will be understood that the overdrive is established within an operating area A defined between the two shift schedule lines and above the curve .alpha., whereas the third gear ratio is established within an operating area B defined between the downshift schedule line (broken line) and below the curve .alpha.. This results in an insufficient acceleration within the operating area A. Besides, the employment of the overdrive does not make as much contribution to the improvement of fuel economy as expected because the overdrive is not used within the operating area B. Desirably, the overdrive should be established within the operating area B and released within the operating area A.